The present invention relates to a servo-brake auto-modulator for a trailer provided with a pneumatic braking system.
For reasons of safety, it has recently been proposed that pneumatic braking systems should be adopted, which have sprung parking-brake members which can be operated by means of a parking distributor interposed in the supply line and operable manually to discharge this line and consequently cause the operation of the sprung brake members. In particular, a pneumatic system for trailers has been considered, in which a servo-auto-modulator is connected to the outlet of the parking distributor and includes, in known manner, a non-return valve for enabling compressed air to flow from the supply line to an accumulator reservoir and a relay valve for applying braking pressure to the service brake members of the vehicle, which is controlled by a braking modulator device in dependence on the load bearing on the vehicle.
The use of a braking system with sprung parking-brake members on a trailer involves the need to prevent the brake members from being overloaded when the brake pedal of the tractor unit and the manual parking distributor are operated simultaneously.
The similar problem which occurs in braking systems for tractor units is currently resolved by the use of so-called "anti-overload" valves or distributors which enable the braking force due to operation of the brake pedal to be cancelled out when the hand brake is operated at the same time.
An anti-overload valve could also be used in a similar manner in a pneumatic system for a trailer which includes sprung parking-brake members.